In locating a control object such as a tool rest of a machine tool, the present invention relates to a method of decelerating a travel driving source of the control object.
To locate a control object, there has conventionally been employed a method which deceleratedly drives a moving driving source of the control object stepwise and stops the control object while allowing it to deceleratedly travel stepwise. However, this method involves a number of problems.
In locating the control object by means of numeric control, for example, locating of the control object is carried out stepwise on the basis of a decelerated travel zone (hereinafter referred to as the deceleration zone) of the control object which is determined in consideration of various characteristics such as a travelling speed of the control object before deceleration, its scheduled moving quantity and the like, and also on the basis of the speed of the control object within the deceleration zone.
According to this prior art method, when the control object reaches the deceleration zone, that is, a first deceleration point, the control object is allowed to travel while being applied with brake by a driving source which is deceleratedly driven by a first stage deceleration instruction, and even when the speed of the control object reaches the first stage deceleration speed, the control object is caused to make uniform motion for a while at the deceleration speed before it reaches a second deceleration point. Thereafter, the decelerated travel required for stopping the control object is repeated in the same way till location control of the control object is realized finally. Hence, this method needs a long period of time before deceleration and stop of the control object are attained in practice. Especially because the control object is allowed to make uniform motion in each deceleration zone at the deceleration speed inherent to each zone, this constitutes one of the main factors that prolong the time required for the location control.
The reason why the stepwise deceleration is generally required as the indispensable element in the location control is that inertias of the control object caused to travel before each deceleration zone, of its driving source and of transmission mechanisms associated with the driving source (e.g., reduction gears and feed screws) must be reduced stepwise so as to reduce or eliminate strain occurring in these mechanical elements, to prevent damage and abrasion of the machine and to improve the location accuracy. However, even according to such a method, it is practically difficult to obtain sufficient effects in preventing damage of the machine and in improving the location accuracy.
Incidentally, from the viewpoint of the prevention of damage of the machine and of the improvement in the location accuracy, there have conventionally been applied various improvements to the machine such as, for example, improvement of the rigidity of the machine itself and use of a ball screw nut in the feed mechanism to minimize the strain occurring during deceleration, but such measures do not function as effective means for preventing damage and abrasion of the machine or fluctuation of the location accuracy due to the stress or strain that occur during the deceleration travel of the machine in the location control.
The inventor of this invention has made intensive studies and experiments to clarify why damage and abrasion of the machine and fluctuation in the location accuracy such as mentioned above occur in the location control and has now found that there is a significant factor in the conventional method of decelerating the driving source for the control object.
In other words, in accordance with the conventional deceleration method, when the control object reaches a given deceleration zone, a control signal is impressed to the driving source to travel the control object in said deceleration zone and to gradually decelerate the control object. Even when the control object attains a predetermined speed of the first deceleration zone, the output of the driving source is maintained as such and the control object is allowed for a while to make uniform motion at the deceleration speed till it reaches the second deceleration zone. Thereafter, the driving source is subjected to the deceleration control in the same way so that the control object, which is sequentially allowed to make uniform motion in the preceeding deceleration zone, is allowed to travel in the state of the same deceleration travel in each of the subsequent deceleration zones. Consequently, at the start of the deceleration in each deceleration zone, there are occurring inertias in the control object, in the driving source allowed to keep such output as to let the control object travel at the predetermined uniform speed and also in the transmission mechanism connecting the control object to the driving source. These inertias occur along with the travel of the control object at the uniform speed in the preceeding deceleration zone and act on the control object, the driving source and the transmission mechanism, thus causing strain of the machine. It has been found that this strain exerts a significant influence on the damage and abrasion of the machine and on the location accuracy. This strain occurs when the control object is caused to travel at the uniform speed by means of the output of the driving source. In other words, since the control object is constantly caused to travel at the uniform speed and at the acceleration (deceleration) or to stop, the strain always occurs in the control object or mechanisms associated with the control object when its travelling state is controlled.